1. Field of the Invention
The invention relates to systems for reducing aerodynamic drag and mitigating detrimental impacts of side wind for land vehicles and, specifically, to aerodynamic skirts for controlling airflow under a vehicle body by self-adaptive aerodynamic means.
2. Description of the Related Art
In the contents of the present invention, a land vehicle is defined as any means for transporting people and/or cargo on the ground. The below description is focused on a specific application of the present invention to cargo vehicles which is done for simplifying explanation of the distinctive physical effects and features of the invention. Any person skilled in the art can apply the described invention to other vehicles.
A standard cargo vehicle consists of a tractor and one or several cargo holding areas (containers) of any kind that are attached to a tractor or towed, referred below as trailers. A vehicle has a body which is disposed above wheel assemblies. As such vehicle moves along its path, the airflow around the vehicle produces aerodynamic drag and increases the fuel consumption. The surrounding airflow may also produce side forces and yawing and rolling moments acting on the vehicle and decreasing its aerodynamic stability and driving safety.
An aerodynamic drag, or air drag, is the aerodynamic resistance force acting on a vehicle and opposing its motion through a surrounding air. It is well-known that about 30% of a vehicle air drag is produced under the vehicle body. Numerous devices typically referred to as skirts have been developed for reducing an aerodynamic drag in the drag-producing volume under the vehicle body at no wind or head/tail wind. However, no special attention was paid to reducing detrimental impacts of a side wind which increases aerodynamic drag at stable or gusty side winds, vehicle jack-knifing and trailer swing, and forces a vehicle out of a traffic lane and may overturn a vehicle by strong gusts. Hereafter a side wind is defined as airflow around a vehicle at non-zero yaw angle.
A diverse body of tractor and trailer skirts has been developed for reducing air drag in the drag-producing volume under bodies of land vehicles and numerous patents for such devices have been awarded around the world. The skirts are typically positioned under a tractor body and/or a trailer body on the driver and passenger sides and configured to form essentially impenetrable walls separating the under-body volume from an external airflow in a lateral direction. Typical embodiments of skirts can be found, for example in the U.S. Pat. No. 7,093,889 to Graham, the U.S. Pat. No. 7,578,541 to Layfield et al., the U.S. Pat. No. 7,740,303 to Wood, the U.S. Pat. No. 7,748,772 to Boivin et al., the U.S. Pat. No. 8,303,025 to Senatro, the U.S. Pat. No. 8,579,359 to Brown et al. and references in those patents. Typical designs of skirts for a tractor cab and trailer can be found, for example in the U.S. Pat. No. D499,679 to Perfetti et al., the U.S. Pat. No. D578,944 to Dolan et al., the U.S. Pat. No. D649,090 to Boivin et al. and references in those patents.
Typical skirts have a height that provides low ground clearance for a vehicle and the skirts are often damaged on uneven roads. To mitigate this problem, flexible and deformable skirts have been developed and typical embodiments can be found, for example in the U.S. Pat. No. 7,686,385 to Dolan et al., the U.S. Patent Application. No. 2011/0233960 to Heinz, the U.S. Pat. No. 8,449,017 to Boivin et al. and references in those patents.
Typical skirts fit only trailers of a height and a separation between tires within limited ranges and adjustable configurations of the skirts have been developed to fit a wide variety of vehicles. Adjustable skirts are often made from separate parts like slats, panels and the like where those parts are joined together for adjusting the skirt length. Although some adjustable skirts consist of separate parts, their assembled operational configuration has an essentially impenetrable surface. Typical embodiments of adjustable skirts can be found, for example in the U.S. Pat. No. 5,921,617 to Loewen et al., the U.S. Pat. No. 6,644,720 to Long et al., the U.S. Pat. No. 7,942,466 to Reiman et al., the U.S. Pat. No. 8,210,599 to Butler, the U.S. Pat. No. 8,398,150 to Brown et al. and references in those patents.
Typical trailer skirts on vehicle sides affect airflow over a significant part of a drag-producing volume under a vehicle body to reduce air drag on several under-body structures such as wheel assemblies, fuel tanks and the like. Some devices were also proposed for reducing air drag on specific structures under the body. For example, underbody fairings for reducing air drag on wheel assemblies are described in the U.S. Pat. No. 7,828,368 to Ortega et al., the U.S. Pat. No. 8,376,450 to Long et al. and references therein.
The existing tractor and trailer skirts have several drawbacks that may discourage their practical implementation. For example, the skirts typically extend almost to the ground and provide a low ground clearance thus interfering with driving on uneven roads. Low-clearance skirts may be damaged or broken thus requiring a substantial maintenance like periodic repairs. Solid skirts without essential gaps are of a heavy weight and also accumulate much dirt, ice and/or snow which degrades their aerodynamic performance and further increases the weight. The majority of existing skirts has pre-determined dimensions varying within limited ranges and do not fit all types of vehicles. Typical existing skirts may be insufficiently effective in reducing air drag at side winds. In particular, essentially impenetrable surfaces of the skirts prevent an ambient air from travelling under a vehicle at side winds which may increase side forces and yawing and rolling moments acting on a vehicle and degrade the vehicle's aerodynamic stability. In addition, existing skirts may be insufficiently effective in reducing air drag on a trailer back.
A need therefore exists for an innovative system that overcomes those shortcomings.